Clarified README a little

Digitization module which translates the collected charges into a digitized signal, emulating a charge sensitive amplifier with Krummenacher feedback.

For this purpose, a transfer function for a CSA with Krummenacher feedback is taken from \[[@kleczek]\]:

```math

H(s) = \frac{R_f}{(1+ \tau_f s) * (1 + \tau_r s)},

H(s) = \frac{R_f}{(1 + \tau_f s) \cdot (1 + \tau_r s)},

```

with fall time constant

```math

```

and rise time constant

```math

\tau_r = \frac{C_{det} * C_{out}}{g_m * C_f}

\tau_r = \frac{C_{det} \cdot C_{out}}{g_m \cdot C_f}

```

The impulse response function of this transfer function is convoluted with the charge pulse. In the time domain, the impulse response function can be written

```math

\mathcal{L}^{-1}(H) = R_f \left( \frac{e^{-t/\tau_f}}{\tau_f - \tau_r} - \frac{e^{-t/\tau_r}}{\tau_f - \tau_r} \right).

```

The impulse response function of this transfer function is convoluted with the charge pulse.

This module can be steered by either providing all contributions to the transfer function as parameters within the `csa` model, or using a simplified parametrization providing rise time and feedback time.

In the latter case, the parameters are used to derive the contributions to the transfer function (see e.g. \[[@binkley]\] for calculation of transconductance).